The Madness of the Jewel King
Ace's ability, The Madness of the Jewel King (宝石王の乱心, Hōseki-ō no Ranshin?), is an ability that turns his subordinates' life (those who equip the collar he gave) into jewels which worth according to the target's lifespan.
According to Kouyou in Chapter 42, Ace gained his position as an Executive by paying the Port Mafia a large sum of money. It is shown that Mori and Kouyou are both conscious of his true self as they put no trust to Ace but Mori allowed him to be in charge of Fyodor's interrogation.
Originally, he was a gambler who laid waste to Port Mafia's casino before he joined Port Mafia.